Interference detection becomes more and more important. In particular, GNSS (global navigation satellite system) signals are extremely susceptible to all types of interference. Therefore the GNSS bands should be constantly monitored to detect possible threats. Moreover, for the same reasons, monitoring of other types of frequency bands may also be used, such as monitoring of UMTS (Universal Mobile Telecommunications System) bands, LTE (Long Term Evolution) bands, FM (frequency modulation) radio broadcasting bands or AM (amplitude modulation) radio broadcasting bands.
In the following, GNSS is considered as an exemplary frequency band. The concepts provided to solve the problems outlined below are equally applicable to other frequency bands, such as a communication signal band (e.g. a telecommunication signal band) using any kind of modulation technique, a radio signal band having a predefined bandwidth and a predefined center frequency, a navigation signal band or a telemetry signal band.
Since the GNSS bands are up to 100 MHz wide, receivers sampling at a Nyquist rate (i.e. at least twice the bandwidth) have very stringent requirements in terms of data rates and data storage.
FIG. 11 shows the L-band spectrum of the current and planned GPS and Galileo global navigation satellite system signals with the notation of their modulation names and carrier frequencies. Signals of a first group of signals are classified, e.g. signals for military purpose only. Furthermore, signals of a second group of signals are open signals. All current and upcoming signals are within the protected Radio Navigation Satellite Services (RNSS) band but only the L1/E1 and L5/E5 bands are within the even better protected spectrum allocated to Aeronautical Radio Navigation Services (ARNS). The other two GNSS bands, E6 and L2, only protected through the RNSS, suffer from radar, military transmissions, and other potentially strong interferers.
Due to the inherently low power of GNSS signals, e.g. approx. −127 dBm received signal power on earth, the GNSS bands are dominated by white Gaussian noise. The noise is about hundred to a few thousand times stronger than the GNSS signal itself. As a consequence, the GNSS signals are extremely susceptible to all types of interference. The interferences can be unintentional like the harmonics of certain oscillators that translate into single continuous wave (CW)-tones or multitones in the GNSS spectrum. Moreover in the E5a/L5 band, the GNSS service is sharing its bandwidth with systems like Distance Measuring Equipment (DME) and Tactical Air Navigation (TACAN) appearing as Gaussian shaped pulse signals. In the L2/E6 band strong military radar signals can appear. But, there are also more and more intentional interferers, so called jammers, readily available on the market, mostly sold over the internet, even if their use is illegal in most countries. The commercial jammers can often be characterized by a chirp signal. All these interferers have in common that a very small output power is sufficient to exceed the thermal noise floor and therefore to effectively affect the GNSS signals.
Since effective interference mitigation techniques, e.g. array processing, or frequency domain adaptive filtering—are mostly unavailable to mass-market GNSS receivers and still relatively uncommon in professional receivers, it is useful to monitor the GNSS bands of interest for later interference detection and elimination.
Different kinds of GNSS bands monitoring networks have already been installed, are currently under development or planned, see, for example, [1], [2], [3].
According to the state of the art, to monitor a GNSS band, basically the complete broadcast bandwidth has to be supervised, e.g. around 50 MHz for Galileo E1, GPS L2 and Galileo E6, 100 MHz for the complete E5 band, see, for example, FIG. 11.
The Nyquist-Shannon sampling theorem states that the sampling rate Fs has to be at least twice the bandwidth of the signal to be digitized in order to avoid aliasing effects. So, according to the state of the art, the complete GNSS signal bandwidth to be monitored is digitized. The useful Nyquist sampling rate of an analog-to-digital converter (ADC) used to digitize a 50 or 100 MHz bandwidth is therefore at least 100 or 200 mega samples per second (MSPS), respectively.
Moreover, the state of the art may use a high dynamic range, since the GNSS signals are around the thermal noise floor power while the interferers can easily reach 80 dB or more. So, a 14 or even 16 bit ADC (analog-to-digital converter) is needed according to the state of the art. The state of the art provides sophisticated 16 bit ADCs with 200 MSPS to digitize the complete E5 band but these ADCs are expensive and have very high power consumption and stringent jitter requirements. They also generate very high data rates resulting in very large files that need to be stored for post-processing.
E.g. the data acquisition system used in [4] to characterize different GNSS jammers features 16-bit I/Q samples with 62.5 MHz resulting in a raw data rate of 2 GBit/s or 250 MByte/s which is already too high for most hard drives for constant data recording. Such a system is not only very expensive but also produces very large amounts of raw data making storage and post-processing very cumbersome.
Thinking about building a regional interference monitoring network, a certain amount of stations have to be present to capture the raw signal and transmit their measurements (e.g. the raw data) to a central server. This server can then process the raw-snapshots, detect and possibly also localize the interference source (see [5]). The high raw-snapshot sizes of current Nyquist sampling data acquisition systems make the data transfer and storage very demanding since e.g. no mobile network connection can be used.
The problems outlined above also exist for other signals than GNSS, such as communication signals (e.g. telecommunication signals) using any kind of modulation technique, e.g. UMTS signals, LTE signals, FM radio broadcasting signals or AM radio broadcasting signals, radio signals having a predefined bandwidth and a predefined center frequency, navigation signals or telemetry signals.